1. Field of the Invention
The present invention relates to a variable gain amplifier used for radio terminals.
2. Description of the Related Art
Radio terminals such as a cellular phone include a variable gain amplifier having variable voltage gain in the transmitter and the receiver. Especially, in code division multiple access (CDMA) systems, more than 80 [dB] variable gain width is required for a variable gain amplifier because many terminals use the same frequency band. Since the variable gain width of an amplifier is from 20 to 30 [dB], it is common to configure the variable gain amplifier by combining a plurality of amplifiers. Furthermore, providing variable gain amplifiers capable of operating with low power consumption is desired, because the power consumption of the radio terminals is limited. For the purpose of reducing the power consumption of variable gain amplifiers, multistage variable gain amplifiers composed of an amplifier having variable voltage gain and an amplifier having variable output current has been proposed.
By decreasing the output current of the amplifiers comprising the variable gain amplifier, the reduction of power consumption of the entire variable gain amplifier is expected. However, in the aforementioned variable gain amplifier, it is not possible to reduce the power consumption of the variable gain amplifier because it is not possible to control the output current of the amplifier having variable voltage gain. In addition, each amplifier having variable voltage gain and the amplifier having variable output current includes a plurality of resistances for generating voltage. When the variable gain amplifier is integrated on a semiconductor chip, the necessity area and the dispersion of resistance value of the resistances creates a problem.